Purpose: This in-phantom study was performed to validate a film technique for measurement of surface dose in isocentric, tangential whole breast irradiation by comparison with Monte Carlo (MC) simulations.
Methods: Solid water phantoms of 20 cm and 30 cm separation were irradiated at 100 cm SAD with lateral half-blocked 6 MV and 15 MV x-ray beams, on a Varian TrueBeam linac. Two centimetres beam clearance on the anterior and inferior phantom edges was used to simulate whole breast tangential field geometry, with a field length of 20 cm and asymmetric field width of 15 cm. Measurements were performed using EBT3 Gafchromicᵀᴹ film on the phantom surface under 0.0 g/cm², 0.03 g/cm² and 0.0995 g/cm² buildup materials to simulate potential breast support devices on the entrance, exit and inferior surfaces of the phantom. Dose was normalized to the mid separation dose. Film calibrations were performed at SAD 100 cm, 10x10 cm² field size at depth of 10 cm for both energies. Surface dose was calculated in depth increments of 0.025 g/cm² from the surface to a depth of 0.1 g/cm² using EGSnrc using the following parameter settings: ECUT=0.521 MeV, PCUT=0.010 MeV, BCA EXACT with a skin depth of 3, ESA PRESTA-II.
Results: Film measurements agreed with MC within 5% at the beam entrance and exit surfaces for both beam energies and all buildup materials. Agreement was with within 10% at the inferior phantom surfaces receiving side scatter from the phantom.
Conclusion: Investigations of this EBT3 film technique to measure surface dose are ongoing. Significant corrections appear not be required when measuring entrance or exit surface dose but further work is needed to improve agreement with MC simulation at the inferior phantom surface, where dose is primarily due to side scatter from the irradiated volume.
Funding Support, Disclosures, and Conflict of Interest: Funding support from the Canadian Cancer Society Research Institute
Not Applicable / None Entered.